The present invention relates in general to a dielectric resonator applicable primarily to microwave bandpass, not limited thereto, and a filter using the dielectric resonator, and more particularly to a 1/4 wavelength multi-stage coaxial resonator of a unitary structure, and a band-pass filter (BPF) and a band-rejection filter (BRF) using such multi-stage coaxial resonator.
Various types of structure of multi-stage filters using high dielectric constant ceramic materials are known. One of the conventional multi-stage filters is shown in FIG. 30A, in which a plurality of (three in FIG. 30A) dielectric rectangular resonators 10 are combined in sidewise coupling arrangement by means of suitable lumped element circuits such as capacitors, coils, etc. In FIG. 30A, the dielectric resonator 10 has a through-hole 14 serving as a resonator hole for resonation at a center of each of the rectangular columns of high dielectric constant material, and a conductive film adhered to outer surfaces of the column except the upper, "open" surface thereof, as well as on an inner surface of the through-hole 14. For the purpose of clarification, the conductive film on the outer surface as above is referred to as an "outer conductor" and the conductive film on the inner surface of the through-hole to a "central conductor". Capacitors C.sub.1, C.sub.2, C.sub.3 are connected to the central conductors at the open surface (upper surface) with coils L.sub.1, L.sub.2 connected between the capacitors. FIG. 30B shows an electric circuit equivalent to the structure of FIG. 30A. The dielectric resonator 10 has its own resonance frequency which is determined by such factors as height or length of the rectangular structure, relative dielectric constant, capacitance of the capacitors applied thereto, and a band-rejection filter of 1/4 wavelength coaxial resonator. An example of the filter characteristics is shown in FIG. 31.
The coupled construction of separate elements as shown in FIG. 30A can be applied to a band-pass filter and yet in a unitary structure as shown in FIG. 32A has been used in general. In the structure of band-pass filter in FIG. 32A, a rectangular parallelopiped dielectric block 16 is provided with three resonance apertures 18 at a predetermined interval and two coupling apertures 20 in an adjoining relation to the resonance aperture, and the outer surfaces, except the upper open surface, and the inner surface of the resonance aperture 18 are provided entirely with, or covered with, a conductive film. Capacitors 22 are coupled to open ends of the resonance apertures positioned at opposite sides of the central resonance aperture for connection with external circuits and devices. An electric circuit equivalent to the structure of FIG. 32A is shown in FIG. 32B, and this band-pass filter has characteristics as shown in FIG. 33. In FIG. 32B three resonator elements 24, coupling capacitors C.sub.01, C.sub.02, at input/output terminals, and coils L.sub.1, L.sub.2 for connecting the resonator elements 24.
The conventional band-rejection filter shown in FIG. 30A consists of a plurality of (three) resonators arranged in a sidewise abutment relation with greater number of parts and elements for assembly and, consequently, increased number of assembly steps is necessary. Thus strict requirements for positioning the resonators and for accuracy of the outer conductive surfaces must be fulfilled. Further, additional requirements for mechanical strength and environmental resistance reliability with respect to the coupling of the resonators must be fulfilled since the resonators must be bonded together.
The band-pass filter of a unitary structure shown in FIG. 32A does not have the disadvantages as described above with respect to the band rejection filter, but has problems that accuracy in dimension and positioning or pitch of coupling apertures and uniformity of a relative dielectric constant must be maintained so as to minimize the influence on the electro-magnetic properties. Therefore, the band-pass filter structure of FIG. 32A provides considerable difficulties in electromagnetic properties and its design.